The invention relates to an electropneumatic drive system which is fed compressed air from a network featuring compressor and compressed air storage facility; the said system is for crust breaking devices which are employed at aluminum fused salt reduction cells and is such that the said drive system comprises at least one working cylinder with piston and piston rod, a slide valve situated after the junction running from the network, valves, compressed air supply lines and a microprocessor. Further, the invention relates to a process for operating the electropneumatic drive system.
In the production of aluminum by fused salt electrolytic reduction of aluminum oxide, the latter is dissolved in a fluoride melt, comprised for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the molten aluminum itself forming the actual cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. As a result of the electrolytic decomposition of the alumina, oxygen is formed at the carbon anodes and combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperture range of about 940.degree.-970.degree. C.
During the electrolytic process the aluminum oxide i.e. the alumina in the electrolyte, is consumed. At a lower concentration of about 1-2 wt. % alumina in the electrolyte the anode effect occurs whereupon the voltage rises from, for example, 4-5 V to 30 V and higher. In production, therefore, modern electrolytic cells are fed with alumina at intervals of only a few minutes, even if no anode effect occurs. To this end it is essential that an appropriate opening in the crust is always available in order that the alumina can be fed in measured quantities to the electrolyte. In the case of modern electrolytic cells therefore the alumina feeding system and crust breaking device are always combined both in terms of location and operation. Under normal operating conditions electronic process control is employed to trigger, e.g. every 2-5 min, the lowering and raising of the breaker chisel on the crust breaker; immediately before or after this the feeding of the alumina takes place. If the anode effect occurs, the frequency is greatly increased.
The lowering of the chisel causes any solidified electrolyte in the opening to be pushed down and redissolved in the melt.
The chisel of the crust breaking device is pneumatically driven almost throughout the whole of its stroke. By means of a mechanically or pneumatically operated stop switch the lowering of the chisel is brought to a halt and its return to the starting position triggered off. The signal for the return of the chisel can, however, also take place via measurement of an electric potential in that on immersion of the chisel in the electrolyte an electric circuit is completed.
In large pot rooms with a hundred or more reduction cells, each of which is fitted with at least one crust breaking device, the enormous quantities of compressed air employed represent a significant cost factor. Of necessity a great deal of energy is required also.